Computer systems are ubiquitous in today's work and home environments. The data stored on these computer systems, however, is vulnerable to theft, fire, flood, and other natural disasters. A data storage system is needed to support a backup and restore system for a large number of client computing devices. Multiple data centers may be employed to store user data backups.
One approach to managing backup data across multiple data centers is to preserve locality of user data backup in one logical data site in a data center. Over time, backup data for existing users continues to grow. In order to relieve space pressure, user data backup needs to be transferred to another logical data site. The transfer of user data backup between logical data sites must be performed with minimal interruption to data backup and restore processes, and without overloading network bandwidth. Thus, a need exists in the art for improved transfer of user data backup between logical data sites.